The manufacturing process causes optical data carriers to have a substrate thickness that locally or globally deviates from the standard thicknesses as prescribed for example in standards for DVD or Blu-ray Disc (BD) optical data carriers. As the objective lens of the optical scanning device is designed for a specific substrate thickness, such deviations introduce spherical aberration in the scanning radiation beam of the optical scanning device when scanning such optical data carriers. The spherical aberration caused by a thickness error is particularly large in BD optical scanning devices because of the high numerical aperture (i.e. 0.85) of the objective lens. In multi-layer BD optical data carriers, e.g. dual-layer BD, a spacer layer separates the information layers on which data is to be recorded or scanned from. Such a spacer layer thickness may cause an additional error in the total thickness to be scanned through. Moreover, even when the manufacturing tolerances of the substrate thickness were very small, the fixed correction provided for in the objective lens would not suit the total thickness of each layer of a dual-layer BD disc. Hence accurate spherical aberration correction is required in order to read and/or write on such a BD disc, especially a multi-layer disc, with sufficient read and or write performance, for example, low jitter.
A known solution of correcting spherical aberration introduced in the scanning beam is a vergence change of the radiation beam towards to objective lens, e.g. by making a parallel radiation beam slightly converging or diverging depending on the sign of the thickness error. Such a change of vergence can be achieved by, for example, the moving-collimator-method: a controlled displacement of a collimator lens in radiation beam towards the objective lens introduces spherical aberration into the scanning beam in order to compensate the spherical aberration introduced by a substrate thickness error or by scanning an information layer at different depth in the optical data carrier than the objective lens was designed for. In order to correct the spherical aberration the actual position of the lens is preferably to be known.
JP2004-272945 discloses an actuator for displacing a collimator lens. The actuator comprises a motor driving a lead screw cooperating with a follower attached to a guided lens holder. The application discloses one or more specific configurations for avoiding errors in positioning when the displacement of the lens holder is reversed in direction.
At startup of for example a BD scanning system applying such a moving collimator, the collimator lens position can be unknown. For this, the collimator lens is displaced to a known location, e.g. a home-position or a stopping-position, at startup. A known solution to detect the home-position is by means of a light-interrupter.
Another method is to move the collimator lens to the home-position with an excessive number of steps of a lead-screw driven by a stepper motor. The collimator lens or holder with collimator lens will hit the home-position, stopping the displacement. A follower nut is attached to the collimator lens or lensholder. The follower nut is cooperating with the lead-screw that is to be driven by the motor. A reference-surface is provided to, for example, the lens holder or follower nut to come into contact with the home-position, thereby determining the position of the collimator lens.
A problem encountered with this method is that when the collimator lens is moved to the home-position the reference-surface can get stuck at this home-position. This is caused by a combination of the composition of the mechanical tolerances, friction variations and friction vectors between the lead-screw, follower nut, motor and home-position. The motor torque required for releasing the collimator lens from the home-position is larger than the torque required for homing. Depending on the available power the motor can apply the reference-surface may remain stuck against the home-position. This may especially occur, when in view of requirement such as, for example, low power consumption and miniaturization the motor has to be small and low power.
In order to avoid the reference-surface to get stuck to the home-position a large pitch of the thread of the lead-screw can be applied, however, this will result in inaccurate position-control of the collimator lens and thus inaccurate control of the spherical aberration correction.
It is an object of the invention to provide an improved actuator for displacement of a holder in which it is avoided that the reference-surface gets stuck against the stopping-surface of the home-position.